1. Field of the Invention
The present invention relates in general to a hydraulically operated system, in particular, a hydraulically operated braking system, and more particularly to such hydraulically operated system which has a mechanically operated and manually controlled hydraulic pressure control device and an electrically controlled hydraulic pressure control device.
2. Discussion of the Related Art
A pressure level of a working fluid used for an actuator can be more easily regulated when the pressure level is electrically controlled by an electrically controlled device than when it is mechanically controlled by a mechanically operated device according an operating force acting on the mechanically operated device. Generally, however, an electrically controlled hydraulic pressure control device is less reliable than a mechanically operated hydraulic pressure control device. In view of these advantage and drawback of the electrically controlled pressure control device, there has been proposed a hydraulically operated system which has a mechanically operated pressure control device and an electrically controlled pressure control device, for regulating the pressure of the working fluid used for an actuator. This hydraulically operated system enjoys the advantage of the electrically controlled pressure control device while compensating or obviating the drawback of the same by using the mechanically operated pressure control device.
The hydraulically operated system of the type indicated above is embodied as a brake system for a motor vehicle, for example. An example of a hydraulically operated brake system of this type is disclosed in JP-A-63-20256, wherein a pressure selecting device is connected to a mechanically operated pressure control device in the form of a master cylinder, an actuator in the form of a wheel brake cylinder for applying braking to a vehicle wheel, and an electrically controlled pressure control device. The pressure selecting device is operated according to a difference between the hydraulic pressures generated by the mechanically operated master cylinder and the electrically controlled pressure control device, so that the electrically controlled pressure produced by the electrically controlled pressure control device is applied to the wheel brake cylinder when the electrically controlled pressure control device is normal, and the manually controlled pressure generated by the master cylinder is applied to the wheel brake cylinder when the electrically controlled pressure control device is not normal. Thus, this brake system has a fail-safe function for operating the wheel brake cylinder in the event of any trouble associated with the electrically controlled pressure control device.
Several types of devices are known as the electrically controlled pressure control device. There will be described three typical types of the electrically controlled pressure control device.
The first type of electrically controlled pressure control device uses a spool valve of two-input type, as disclosed in the above-identified publication JP-A-63-20256, which includes a spool for selective fluid communication of the wheel brake cylinder with a hydraulic pressure source and a reservoir. The spool is adapted to receive an electrically controlled magnetic drive force and the hydraulic pressure in the wheel brake cylinder (hereinafter referred to as "brake cylinder pressure"), such that the magnetic drive force and a force based on the brake cylinder pressure act on the spool in the opposite directions. The brake cylinder pressure is regulated by the spool depending upon the electrically controlled magnetic drive force, so as to maintain equilibrium between the magnetic drive force and the force based on the brake cylinder pressure. The brake cylinder pressure which is thus electrically controlled by the spool valve is applied to the wheel brake cylinder.
The second type of electrically controlled pressure control device uses a spool valve of three-input type as disclosed in unexamined Japanese Patent Application No. 5-120686 (which was filed on Apr. 22, 1993 and has not been laid open at the time of filing of the present application). This spool valve includes a spool as used in the first type, but this spool receives the master cylinder pressure or a pressure (hereinafter referred to as "equivalent master cylinder pressure") substantially equal to the master cylinder pressure, as well as the brake cylinder pressure and the electrically controlled magnetic drive force, such that the brake cylinder pressure and the equivalent master cylinder pressure act on the spool in the opposite directions. The brake cylinder pressure is regulated by an operation of the spool so as to maintain equilibrium between the force based on the brake cylinder pressure and a sum of the magnetic drive force and a force based on the equivalent master cylinder pressure. Described more specifically, the brake cylinder pressure is regulated depending upon the equivalent master cylinder pressure when no magnetic drive force acts on the spool, and depending upon the sum of the magnetic drive force and force based on the equivalent master cylinder pressure when the electrically controlled magnetic drive force acts on the spool. In this type of device, the magnetic drive force is negative or positive. Namely, when the magnetic drive force is negative or acts on the spool in a direction opposite to the direction in which the equivalent master cylinder pressure acts on the spool, the spool valve functions as a pressure reducing valve which regulates the brake cylinder pressure (as the electrically controlled pressure) within a pressure range lower than a standard pressure range which is a range in which the brake cylinder pressure is regulated when the magnetic drive force is not applied to the spool. When the magnetic drive force is positive or acts on the spool in the same direction as the equivalent master cylinder pressure, the spool valve functions as a pressure booster valve which regulates the brake cylinder pressure within a range higher than the standard standard pressure range.
The third type of electrically controlled pressure control device uses a shut-off valve which includes a valving member operated by an electric current. The valving member has at least a pressure-increase position in which the wheel brake cylinder is in communication with the hydraulic pressure source and is disconnected from the reservoir, and a pressure-decrease position in which the wheel brake cylinder is in communication with the reservoir and is disconnected to the hydraulic pressure source. The brake cylinder pressure is raised and lowered when the valving member is placed in the pressure-increase and pressure-decrease positions, respectively. The valving member may have a pressure-hold position in which the wheel brake cylinder is disconnected from both of the reservoir and hydraulic pressure source, to hold the brake cylinder pressure.
The electrically controlled pressure control devices as described above suffer from relatively low pressure control capability particularly where there are restrictions or limitations on electric power available for the device. In the case of the electrically controlled pressure control device using the spool valve of two-input type discussed above, for example, the limited electric power consumption leads to difficulty to maximize the level of the electrically controlled pressure (e.g., brake cylinder pressure) obtained by the device. In the case of the device using the spool valve of three-input valve also discussed above, the limited electric power consumption creates problems such as insufficient reduction of the brake cylinder pressure when the spool valve functions as a pressure reducing valve, and insufficient boosting of the brake cylinder pressure when the spool valve functions as a pressure booster valve. The problem of the insufficient pressure reduction will be explained in detail, with respect to the preferred embodiments of the invention. In the case of the device using the shut-off valve discussed above, it is difficult to construct the shut-off valve with a sufficiently large cross sectional area of flow of the working fluid (brake fluid) so as to assure a sufficiently small resistance of the fluid flow through the shut-off valve. In an attempt to solve these problems, the assignee of the present application proposed a technique as disclosed in laid-open Publication No. 1-76363 of unexamined Japanese Utility Model Application, wherein a pressure boosting device is interposed between the electrically controlled pressure control device and the actuator, so that the pressure boosting device reduces the burden or role of the electrically controlled pressure controlled device.
The provision of the pressure selecting device and the pressure boosting device which have been described above assures only only improved hydraulic pressure control capability of the electrically controlled pressure control device, and the fail-safe operation of the brake system in the event of an electrical trouble with the system. However, the pressure selecting device and pressure boosting device constructed as separate structures inevitably increase the size of the hydraulically operated system as a whole.